Data are transmitted via a broadband channel in broadband wireless communications networks employing multiple access methods such as time division duplex (TDD) Code Division Multiple Access (CDMA) and TDD Orthogonal Frequency Division Multiple Access (OFDMA). A broadband channel can be divided into a plurality of sub-channels, each of which comprises a frequency band with distinct channel characteristics.
In a TDD CDMA or OFDMA network, the channel characteristics of downlink channels may be different from those of uplink channels. One of the reasons is that downlink channels and uplink channels are assigned in different frequency bands for a specific user. Another reason is that there is frequency mismatch between downlink channels and uplink channels. As a result, a base transceiver station (BTS) has no means to retrieve downlink channel information from messages transmitted via uplink channels. More specifically, it is difficult for a BTS to generate downlink beamforming weighting vectors for downlink channels without explicitly receiving channel information about the downlink channels. However, it is undesirable for a BTS to explicitly send channel information about downlink channels as the transmission of channel information consumes bandwidth. Therefore, it is crucial for a wireless communications network to be able to employ a method that can generate downlink beamforming weighting vectors in the case of frequency mismatch between downlink channels and uplink channels for a specific user.
In a TDD network, an uplink channel coefficient matrix is strongly correlated with a downlink channel coefficient matrix. Therefore, an uplink channel coefficient matrix can be transformed into a downlink channel coefficient matrix using the correlation between the two matrices. Specifically, downlink beamforming weighting vectors can be generated by using the uplink channel coefficient matrix.